A practical concept for catalytic carbonylations using carbon dioxide

The rise of CO2 in atmosphere is considered as the major reason for global warming. Therefore, CO2 utilization has attracted more and more attention. Among those, using CO2 as C1-feedstock for the chemical industry provides a solution. Here we show a two-step cascade process to perform catalytic carbonylations of olefins, alkynes, and aryl halides utilizing CO2 and H2. For the first step, a novel heterogeneous copper 10Cu@SiO2-PHM catalyst exhibits high selectivity (≥98%) and decent conversion (27%) in generating CO from reducing CO2 with H2. The generated CO is directly utilized without further purification in industrially important carbonylation reactions: hydroformylation, alkoxycarbonylation, and aminocarbonylation. Notably, various aldehydes, (unsaturated) esters and amides are obtained in high yields and chemo-/regio-selectivities at low temperature under ambient pressure. Our approach is of interest for continuous syntheses in drug discovery and organic synthesis to produce building blocks on reasonable scale utilizing CO2.

GC analysis for the gas phase was performed using Agilent HP-PLOT/Q fitted with TCD (thermal conductivity detector) and FID (flame ionization detector) detectors. Carbonylation related GC analysis was performed on a Trace 1310 chromatograph with a 29 m HP5 column. The products were measured by MS and GC analysis or by isolation from the reaction mixture by solvent evaporation and further purified by column chromatography on silica gel.
BET surface area and pore volume of the prepared catalysts were measured from nitrogen adsorption isotherms measured at -196 °C (Micromeritics ASAP 2010). Before the measurement, each sample was degassed at 200 °C for 4 h. The average pore diameters were calculated from the desorption branch of the isotherm using the BJH method. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis was performed using Varian/Agilent 715-ES analyzer.
XRD powder patterns were recorded on a Stoe STADI P diffractometer, equipped with a linear Position Sensitive Detector (PSD) using Cu K radiation (λ = 1.5406 Å). Processing and assignment of the powder patterns was done using the software WinXpow (Stoe) and the Powder Diffraction File (PDF) database of the International Centre of Diffraction Data (ICDD).
For the TPR experiments, the measurement was done using a Micromeritics Autochem II 2920 instrument. A 100 mg sample was loaded in U shaped quartz reactor and heated from RT to 400°C with 20K/min in 5% O2/He (50 ml/min) for 30 min at 400 °C, then flushing and cooling down to RT under the flow of Ar. The TPR measurement was carried out from RT to 700 °C (holding time 30 min) in a 5% H2/Ar flow (50 ml/min) with a heating rate of 10 K/min. Another TPR measurement was done as described previously only in the pretreatment step Ar was used rather than O2. The hydrogen consumption peaks were recorded with temperature using thermal conductivity detector. Quantitative analysis of the TPR data was calculated based on the peak areas.
The TEM measurements were performed at 200kV with an aberration-corrected JEMARM200F (JEOL, Corrector: CEOS). The microscope is equipped with a JED-2300 (JEOL) energydispersive x-ray-spectrometer (EDXS) and an Enfinum ER (GATAN) with Dual EELS for chemical analysis. The solid samples were deposed without any pretreatment on a holey carbon supported Ni-grid (mesh 300) and transferred to the microscope. XPS data was obtained with a VG ESCALAB220iXL (ThermoScientific) with monochromatic Al Kα (1486.6 eV) radiation. Binding energies were corrected to C-C contribution at 284.8 eV in C1s region. For quantitative analysis, the peaks were deconvoluted with Gaussian-Lorentzian curves, the peak area was divided by a sensitivity factor obtained from the element specific Scofield factor and the transmission function of the spectrometer.
For the characterization of the Cu species CO was used as probe molecule. In situ FTIR spectroscopic measurements in transmission mode were carried out on a Bruker Tensor 27 FTIR spectrometer equipped with a heatable and evacuable homemade reaction cell with CaF2 windows connected to a gas-dosing and evacuation system. The sample powders were pressed into self-supporting wafers with a diameter of 20 mm and a weight of 50 mg. The samples were pretreated by heating in vacuum up to 400°C and keeping at this temperature for 1h. After dosing CO2/H2=1:3 for 5 min the reaction cell was closed, and the reaction was monitored for 60 min. After cooling to room temperature and evacuation the sample was exposed to 5% CO/He. The CO adsorbate spectrum was recorded after removing the gas phase by evacuation the cell.
The reaction set-up was shown in Fig. S1. Catalyst preparation for selective generation of CO from CO 2 /H 2 in Step I.
The catalysts Cu@supports-PHM were prepared using precipitation-hydrothermal method (PHM). In a typical synthesis process, desired amount of metal precursor was dissolved in 20 mL of DI H2O to obtain different metal loadings and stirred for 15 min. 2.5 ml of 28-30% Aq. NH3 solution was added dropwise under continuous stirring for 30 min. Then the support (SiO2, Al2O3, or C) was added to the precipitate and the mixture was stirred vigorously for 4 h at room temperature. The contents were then transferred to a 100 mL autoclave and heated at 120 °C for 8 h without stirring. The autoclave was then allowed to cool naturally to room temperature and the slurry was centrifuged and washed several times with ethanol and dried at 80 °C for 10 h. The dried material was then pyrolysed at 600 °C for 2 h under Ar atmosphere. For comparison, the catalyst 10Cu@SiO2-CIM was also prepared using conventional impregnation method. Here the silica support was directly added to metal nitrate solution and stirred for 4 h followed by drying at 80 °C for 10 h and pyrolysis in Ar at 600 °C for 2 h.

The Step I catalyst characterization: BET
The physiochemical properties of all the prepared and tested catalysts were studied using ICP and BET analysis as listed in Table S1. The ICP analysis showed that the actual metal loading was close to the initial designed value. The surface area of all the samples decreased upon metal loading as compared to blank silica support which was understandable due to the blockage of pores by metal particles which was also evident from the decrease in the total pore volume.

The Step I catalyst characterization: XRD
The XRD analysis was used to study the characteristics of different crystalline and amorphous phases of the samples (Fig. S2). The blank silica support had a highly amorphous phase with a broad peak at 2θ = 22.5° (JCPDS database PDF: 00-039-1425, SiO2-cristobalite). The XRD pattern of the 10CIM sample showed diffraction peak of highly crystalline CuO (tenorite JCPDS: 00-041-0254) indicating the formation of large metal particles. 1-2 On the contrary, the sample prepared using PHM had no CuO peak which indicates that it is either present in the reduced state (Cu2O/Cu 0 ) or is highly dispersed over the support with small nanoparticles which could not be detected using XRD analysis. Moreover, two other distinct metallic phases of Cu2O (cubic JCPDS: 01-071-3645) and Cu 0 (cubic JCPDS: 01-071-4610) were observed. [3][4] Fig. S2 Catalysts XRD characterization. XRD patterns of SiO2, 10Cu@SiO2-PHM, and 10Cu@SiO2-CIM samples.

The Step I catalyst characterization: H 2 -TPR
The H2-TPR (Fig. S3a) was performed to study the metal oxide reducibility of the different prepared samples after pretreatment under 5% O2/He. The 10Cu@SiO2-PHM sample showed a single low temperature reduction peak indicating the presence of only one type of finely dispersed CuO species with no large CuO clusters. On the contrary, the 10Cu@SiO2-CIM sample has two distinct peaks which can be assigned to two different types of Cu species. The low temperature reduction peak at 250 °C can be attributed to smaller and well dispersed CuO, while a broad high temperature reduction peak at 410 °C can be ascribed to larger and poorly dispersed CuO crystallites. It has been previously established that Cu 2+ species strongly interacting with the support are more difficult to be reduced as compared to CuO in bulk. [5][6][7][8] The results are in well agreement to the XRD analysis of CIM catalyst which shows the formation of CuO with large crystalline size. To further identify the nature of Cu species in our samples, TPR experiments (Fig. S3b-c) were performed using Argon pre-treatment (30 min at 200 °C) instead of O2. Then the sample was cooled down to room temperature followed by TPR analysis by heating the sample under the flow of 5%H2/Ar, 10K/min up to 700 °C and kept for 30 min at 700 °C. From these measurements, the experimental amount of H2 consumption of an inactive 10Cu@SiO2-CIM sample was 1290.8 µmol/g (ICP = 8.3 is 1307 µmol/g), indicating that it contains only Cu 2+ which was almost completely reduced to Cu0 at higher temperature (390 °C). However, our active 10Cu@SiO2-PHM showed 627.97 µmol/g (ICP = 9.1 is equal to 1433.1 µmol/g) H2 consumption which indicates the presence of a mixture of Cu 1+ /Cu 2+ . Additionally, the active sample reduction profile starts at lower temperature (104°C, Fig. S3b) in comparison with 10Cu@SiO2-CIM which only shows a minor hump at the lower reduction temperature. General procedure for reducing CO 2 with H 2 into CO catalyzed by 10Cu@SiO 2 -PHM: Investigation of temperature and pressure.
The reaction for reducing CO2 with H2 into CO was carried out in a fixed bed flow-reactor (8.9 mm ID). The catalyst bed at the center of the reactor was supported by glass wool and quartz sand (dilution 1:5). Temperature was measured by a K-type thermocouple inserted into the catalyst bed.
A gas mixture of H2 and CO2 (H2:CO2= 3:1) at a total flow rate of 100 NmL/min was fed into the reactor (0.3 g catalyst, 0.25-0.42 mm, GHSV = 15,000 h -1 ). The volumetric flow rate of the feed gases was controlled by pre-calibrated mass flow controllers (Brooks Instrument). The reaction temperature was increased from 200 to 400°C. The CO2 conversion and product selectivity are defined as follows: where mCO2(in) and mCO2(out) are the moles of CO2 in and out of the reactor. The selectivity is defined as the percentage of moles of CO2 consumed to form desired product (CO, CH4 or MeOH), in respect to the amount of CO2 consumed during the reaction. Various carbonylation reactions using CO 2 /H 2 as CO source. Under argon atmosphere, a 25 mL three-necked round bottom flask was charged with Rh(acac)(CO)2 (0.1 mol%), monodentate ligand (0.4 mol%) or bidentate ligand (0.2 mol%), and oven dried stirring bar. The flask was assembled with a -20°C condenser and a pipeline which connect to the fixed bed flow-reactor. The water generated during the CO2 reduction (step I) was not removed and the resulting gas mixture (CO2, H2, CO) was constantly bubbled through the reaction solution at flow-rate of 100 ml/min. 1a (10 mmol, 1.6 mL) and THF (20.0 mL) were injected into the flask by syringe. The flask was then sealed with cap, the condenser was connected to the ventilation system and the flow-reactor was opened for bubbling in the solution. The GC analysis was performed while taking a gas sample directly from the reaction flask. Gas composition (%) analysis by GC of the gas mixture (H2 : CO : CO2 = 70 : 9.0 : 21). The reaction was performed for 20 h at 25 °C. After the reaction finished, the yield and l/b selectivity were determined by GC analysis using isooctane as the internal standard.  General procedure for hydroformylation of various alkenes using CO 2 /H 2 as CO source.
Under argon atmosphere, a 25 mL three-necked round bottom flask was charged with Rh(acac)(CO)2 (1.3 mg, 0.1 mol%), 6-diphenylphosphino-2-pyridone (L1, 5.6 mg, 0.4 mol%), and oven dried stirring bar. The flask was assembled with a -20°C condenser and a pipeline which connect to the fixed bed flow-reactor. Alkene (5.0 mmol) and THF (20.0 mL) were injected into the flask by syringe. The flask was then sealed with cap, the condenser was connected to the ventilation system and the flow-reactor was opened for bubbling in the solution. The reaction was performed for 20 h at 25 °C. After the reaction finished, the l/b selectivity was determined by GC analysis, and the product was isolated with column chromatography.
General procedure for methoxycarbonylation of 3a using CO 2 /H 2 as CO source: Investigation of ligands.
Under argon atmosphere, a 25 mL three-necked round bottom flask was charged with Pd(acac)2 (15 mg, 1.0 mol%), monodentate ligand (4.0 mol%) or bidentate ligand (2.0 mol%), paratoluenesulfonic acid monohydrate (PTSA▪H2O, 76 mg, 8.0 mol%) and oven dried stirring bar. The flask was assembled with a -20°C condenser and a pipeline which connect to the fixed bed flow-reactor. Phenylacetylene (3a, 0.55 mL, 5.0 mmol), and MeOH (10.0 mL) were injected into the flask by syringe. The flask was then sealed with cap, the condenser was connected to the ventilation system and the flow-reactor was opened for bubbling in the solution. The reaction was performed at 25 °C. After 20 h, the reaction was stopped, the yields and selectivity were determined by GC using isooctane as internal standard.

General procedure for Pd-catalyzed methoxycarbonylation of various alkynes with CO 2 /H 2 as CO source.
Under argon atmosphere, a 25 mL three-necked round bottom flask was charged with Pd(acac)2 (15 mg, 1.0 mol%), 2-(diphenylphosphino)pyridine (L8, 53 mg, 4.0 mol%), para-toluenesulfonic acid monohydrate (PTSA▪H2O, 76 mg, 8.0 mol%) and oven dried stirring bar. The flask was assembled with a -20°C condenser and a pipeline which connect to the fixed bed flow-reactor. Alkynes (3, 5.0 mmol), and MeOH (10.0 mL) were injected into the flask by syringe. The flask was then sealed with cap, the condenser was connected to the ventilation system and the flowreactor was opened for bubbling in the solution. The reaction was performed at 25 °C. After 20 h, the reaction was stopped, the selectivity was determined by GC analysis, and the product was isolated with column chromatography.
General procedure for aminocarbonylation of aryl iodide using CO 2 /H 2 as CO source.
Under argon atmosphere, a 25 mL two-necked round bottom flask was charged with Pd(dba)2 (17 mg, 0.03 mmol, 1.0 mol%), PPh3 (16 mg, 0.06 mmol, 2.0 mol%), aryl iodide (3 mmol), and oven dried stirring bar. The flask was assembled with a -20°C condenser and a pipeline connected to the fixed bed flow-reactor. Toluene (10 mL), Et3N (0.84 mL, 6.0 mmol, 2.0 equiv.) and amine (6.0 mmol, 2.0 equiv.) were injected into the flask by syringe. The flask was then sealed with cap, the condenser was connected to the ventilation system and the flow-reactor was opened for bubbling in the solution. The reaction was stirred for 20 h at 80 °C. After 20 h, the conversion and selectivity were determined by GC analysis, and the product was isolated with column chromatography.